Secondary electron emitter and method of making it



Feb. 20, 1940. H. BRUINING ET AL 2,190,695

SECONDARY ELECTRON EMITTER AND METHOD OF MAKING IT Fi led March 15, 1938 1 conr 543E wm/ ca/voucrwz LAYER 0F CARBUN l-XPOSE CARBON COATED 2 BASE r0 VAPOR/ZED CAES/UM nv 020550 VESSEL AND DEPOSIT CAESIUM o/v CARBON LA YER 3 OXIDIZE MES/UM 0N CARBON LAYER HVVENTORS HAJO BRl/INING, JAN HENDRIK DE BOERAND ARTEN CORNELL? TEVES ATTORNEY.

Patented Feb. 20, 1940 UNITED STATES PATENT OFFICE,

SECONDARY ELECTRON EMITTER AND METHOD OF MAKING IT Application March 15,

1938, Serial No. 195,941

In the Netherlands April 2, 1937 4 Claims.

This invention relates to a method of manufacturing secondary-emission electrodes, i. e.

electrodes consisting, at least on part of their surface, of a material that readily emits secondary electrons when it is struck by primary electrons.

The invention relates furthermore to electric discharge tubes comprising such an electrode.

Various materials have previously been proposed as coatings of such secondary emission electrodes; amongst them an important place is held by the metals and compounds, more particularly the oxides, of the alkali metals and in particular of caesium. A very customary method of manufacturing, for example, a secondary emission electrode which contains caesium consists in' that within an exhausted tube caesium is applied by vaporisation to an oxidised base or underlayer and then is oxidised and activated. A drawback of this method is that upon being vaporised the caesium deposits not only on the surface of the secondary-emission electrode but also at other points of the discharge tube, for example on a primary cathode, at which points the presence of caesium may have a harmful effect.

This drawback may be avoided by utilising a method according to the present invention which consists in that in a closed discharge tube caesium is vaporised and is precipitated on a layer of carbon provided on a base or underlayer and is 30 subsequently oxidised and is present in finely divided condition in and on the layer of carbon. It has been found that such an electrode possesses excellent properties for secondary emission and also that the manufacture thereof has no harm- 5 ful effect on other parts of the tube since owing to the adsorbing and combining action of the layer of carbon the caesium deposits at the desired place. This layer of carbon forms at the same time an underlayer having sufi'icient proper- 4o ties of conduction.

In the accompanying drawing Figure 1 shows the steps of the method according to the invention and Figure 2 shows in greatly enlarged crosssection a portion of an electrode made in accordance with the invention.

In Figure 1 the rectangles numbered I, 2, and 3 represent the successive steps of the method. The finished electrode shown in Figure 2 comprises a base 4 on which is a conductive layer 5 of car- 50 .bon deposited on the base by any of various well known methods. The conductive layer of carbon has in and on it an oxide such as caesium oxide which is present in finely divided condition and the particles of which are indicated by the dots 6.

The caesium is precipitated on the layer of carbon from caesium vapor in a closed discharge tube and the caesium oxide is produced by subsequently oxidizing the precipitated caesium.

Although the invention has been described with reference to one of the alkali metals it may be applied as well to the other alkali metals viz. sodium, potassium, lithium and rubidium. Caesium is, however, particularly suitable for being applied with the aid of the method according to the invention since this metal is most readily adsorbed and taken up by carbon. It has proved to be possible to obtain in this way secondaryemission electrodes of which the secondary emission ratio varies between 3 and 4 at 500 volts. In some cases, more particularly when operating with a comparatively large quantity of caesium,

a secondary emission of 4 secondary electrons per primary electron may be obtained already at 150 volts.

What we claim is:

1. The method of making a surface having secondary electron emissivity several times unity which consists in providing a body with a conductive layer of carbon, depositing alkali metal from the vapor phase on the surface of said carbon layer and oxidizing the alkali metal deposited on the carbon surface.

2. The method of making a surface having secondary electron emissivity several times unity which consists in exposing a body having a conductive surface layer of carbon to the vapor of an alkali metal, depositing said alkali metal from the vapor phase on said carbon surface, exposing the electrode to an oxidizing atmosphere and converting the deposited alkali metal into oxide.

3. The method of making a surface having secondary electron emissivity several times unity which consists in enclosing an electrode having a carbon surface in an evacuated vessel, depositing caesium from the vapor phase on the carbon surface of said electrode, introducing an oxidizing atmosphere into said vessel, and converting the deposited caesium into caesiun oxide.

4. An electrode having a/fatio of secondary electron emission of several times unity upon im-'- pact by primary electrons at several hundred volts and comprising a layer of carbon in and on which caesium oxide is present in finely divided condition.

HAJO BRUINING. JAN HENDRIK nr: BOER. MARTEN CORNELIS TEVES. 

